Chronic itch or pruritus represents a significant clinical problem for which no effective drugs are available. Molecular and cellular mechanisms of chronic pruritus are not well understood and understudied. Recent identification of the first itch-specific gene, gastrin-releasing peptide receptor (GRPR), in the spinal cord opens the way for studying molecular and cellular basis of itch sensation in the nervous system. The goal of this revision supplement is to characterize the identity of gastrin-releasing peptide (GRP)- and its receptor GRPR- expressing cells in the skin, the spinal cord and the brain, respectively. We also plan to examine GRP fibers in chronic pain states. To this end, GRP-eGFP mice and GRPR-eGFP-creERT2 mice will be generated by a knock-in approach. The innervation of GRP+ fibers in the epidermis will be analyzed by immunocytochemistry, whereas whether GRPR-eGFP neurons project to the brain will be determined by retrograde labeling technique. The molecular identity of GRPR+ cells will also be analyzed by immunocytochemistry and in situ hybridization. The proposed studies should provide excellent genetic and molecular tools essential for us to visualize GRP/GRPR- related itch neuronal pathways. Characterization of GRP+ or GRPR+ expressing cells should greatly expedite the tempo of the research on chronic itch. PUBLIC HEALTH RELEVANCE: Chronic itch or pruritus represents a significant clinical problem for which no effective drugs are available. To understand the neural mechanisms of itch, the neuronal pathways that express the itch receptor, GRPR, and its ligand GRP, will be studied by the generation of GRP-eGFP and GRPR-eGFP-creERT2 mice. The proposed studies will advance our understanding of both central and peripheral neural systems important for itch sensation.